Transmission assembly

ABSTRACT

A drive device incorporating a hydraulic pump having a swash plate. At least two structural ribs and skirting ribs are located on and extend along the length of the swash plate. A plurality of thrust ribs may be formed on an inner housing surface and adjacent to respective contact surfaces on the swash plate, the thrust ribs being located so that at least one, but fewer than all, of the ribs can be in contact with its respective contact surface on the swash plate at a time, permitting rotation while limiting axial movement of the swash plate. A trunnion shaft includes a free end disposed so as to be accessible from outside the housing through a bore formed therein, and a control arm having a shaft portion extends into and supports the distal free end of the trunnion shaft within the bore.

This patent application is a continuation of, and claims priority to andthe benefit of, U.S. patent application Ser. No. 13/336,321, filed onDec. 23, 2011, which claims priority to and the benefit of U.S.Provisional Patent Application No. 61/427,002, filed on Dec. 23, 2010,the entire contents of each of which are incorporated herein byreference.

BACKGROUND OF THE INVENTION

This application relates to hydrostatic transmissions generally and, inparticular, to transmissions intended for use in driving a vehicle orother apparatus, such as a walk-behind leaf blower, snow thrower, brushcutter or various types of lawn mower.

SUMMARY OF THE INVENTION

An improved transmission assembly is disclosed herein, as described inmore detail below. The transmission assembly may be mounted on a vehicleor other powered machine or apparatus, such as a leaf blower or lawnmower, for example, in various orientations to accommodate a variety ofvehicle or equipment configurations.

A better understanding of the objects, advantages, features, propertiesand relationships of the invention will be obtained from the followingdetailed description and accompanying drawings which set forthillustrative embodiments that are indicative of the various ways inwhich the principles of the invention may be employed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a transmission assembly in accordancewith the present invention.

FIG. 2 is a perspective view of the transmission assembly of FIG. 1rotated 180 degrees about the input shaft axis and with the fan andpulley assembly removed from the input shaft.

FIG. 3 is a perspective view of the transmission assembly of FIG. 2 withthe main housing removed.

FIG. 4 is a partially exploded perspective view of the transmissionassembly of FIG. 2.

FIG. 5 is a perspective view of an alternative housing cover plate.

FIG. 6 is a perspective view of the transmission assembly of FIG. 1 withthe main housing and various other components removed for clarity.

FIG. 7 is a top plan view of the transmission assembly of FIG. 2 withalternative or additional structure shown in phantom.

FIG. 8 is a cross-sectional view of the transmission assembly of FIG. 7along line 8-8.

FIG. 9 is an enlarged detail view of the sealing interface between theswash plate trunnion shaft and the control arm as indicated by circle 9of FIG. 8.

FIG. 10 is a perspective view of the center section of the transmissionassembly of FIG. 1.

FIG. 11 is a perspective view of the center section of FIG. 10 rotated180 degrees about the input shaft axis.

FIG. 12 is a bottom plan view of the main housing of the transmissionassembly of FIG. 1.

FIG. 13 is a bottom plan view of the transmission assembly of FIG. 1with the housing cover plate removed.

FIG. 14 is a side elevational view of a walk-behind leaf blowing machinewith a portion of the frame cut away and one wheel removed for clarity,the leaf blowing machine incorporating the transmission assembly of FIG.1 with an alternative mounting lug location as illustrated in FIG. 7.

FIG. 15 is a side elevational view of a walk-behind mowing machine witha portion of the frame cut away and one wheel removed for clarity, themowing machine incorporating the transmission assembly of FIG. 1 with analternative mounting lug location as illustrated in FIG. 7.

FIG. 16 is a side plan view of a portion of an alternative bypass armmechanism.

FIG. 17 is a side plan view identical to FIG. 16, except with the bypassmechanism engaged.

FIG. 18 is a cross-sectional view of the bypass arm mechanism takenalong line 18-18 of FIG. 17 with the housing shown in phantom.

FIG. 19 is a partial, exploded perspective view of an alternativetrunnion shaft and control arm assembly.

FIG. 20 is a partial, side plan view of the trunnion shaft shown in FIG.19.

FIG. 21 is a partial, side plan view of the control arm shown in FIG.19.

FIG. 22 is a perspective view of an alternative expansion bladder.

FIG. 23 is a bottom plan view of the bladder shown in FIG. 22.

DETAILED DESCRIPTION OF THE DRAWINGS

The description that follows describes, illustrates and exemplifies oneor more embodiments of the present invention in accordance with itsprinciples. This description is not provided to limit the invention tothe embodiments described herein, but rather to explain and teach theprinciples of the invention in order to enable one of ordinary skill inthe art to understand these principles and, with that understanding, beable to apply them to practice not only the embodiments describedherein, but also other embodiments that may come to mind in accordancewith these principles. The scope of the present invention is intended tocover all such embodiments that may fall within the scope of theappended claims, either literally or under the doctrine of equivalents.

It should be noted that in the description and drawings, like orsubstantially similar elements may be labeled with the same referencenumerals. However, sometimes these elements may be labeled withdiffering numbers, such as, for example, in cases where such labelingfacilitates a more clear description. Additionally, the drawings setforth herein are not necessarily drawn to scale, and in some instancesproportions may have been exaggerated to more clearly depict certainfeatures. As stated above, the present specification is intended to betaken as a whole and interpreted in accordance with the principles ofthe present invention as taught herein and understood by one of ordinaryskill in the art.

Referring to FIGS. 1-13, variable speed hydrostatic transmissionassembly 30 generally comprises an input shaft 35 driving an axialpiston pump assembly 40 rotatably disposed on a pump running surface 38a of a center section 38, the center section comprising hydraulic fluidports and passages to hydraulically drive an axial piston motor assembly42, the motor assembly 42 rotatably disposed on a motor running surface38 b of the center section 38 and driving an output shaft 36. The designof the axial piston pump and motor assemblies of transmission assembly30, comprising pistons disposed in a rotatable cylinder block, arewell-known in the art and will not be described in detail herein.

Hydrostatic transmissions of this type are described generally in, e.g.,commonly-owned U.S. Pat. No. 5,314,387. A hydrostatic transaxle designused in a snow thrower is shown in commonly-owned U.S. Pat. No.6,651,529. The terms of both of these patents are incorporated herein byreference.

The illustrated embodiment of transmission assembly 30 is a sealed unitwhich may be oriented in any position, depending on the vehicle orequipment configuration to which it is applied. However, for thepurposes of this description, position and orientation terms such astop, bottom, upper, lower, vertical, horizontal, etc., will be appliedto transmission assembly 30 as shown in FIG. 8 unless otherwise statedherein.

Referring to FIGS. 4 and 8, transmission assembly 30 comprises internalsump 55 formed primarily by joining and sealing a main housing 32 and ahousing cover plate 33 along a horizontal split line by means of aplurality of screws 37 (or other suitable fasteners) and known sealingmethods, such as an adhesive-sealant or a gasket. Installation of aninternal sump fluid expansion bladder 50 with bladder cover plate 51further serves to form sump 55, as does the installation of variousshaft seals such as input shaft seal 56, output shaft seal 57, trunnionshaft seal 47, control arm shaft seal 48 and bypass shaft seal 72.Bladder cover plate 51 includes one or more vent holes 51 a to allow airto flow in and out of the pocket 60, formed by bladder 50 and bladdercover plate 51, as the fluid in sump 55 expands and contracts during theoperating cycle of transmission assembly 30. An optional open cellstructure foam block (not shown) may be installed in pocket 60, ifneeded, to help control the shape of the collapsing bladder 50 as thesump 55 fluid temperature rises.

As shown most clearly in FIGS. 4 and 8, expansion bladder 50 is insertedpartially through opening 33 b of cover plate 33 so that flange 50 a ofexpansion bladder 50 fits into sealing groove 33 d formed in cover plate33 to create a seal around the perimeter of expansion bladder 50. Flange50 a is then trapped and retained when bladder cover plate 51 isinstalled. Both the upper and lower surfaces of flange 50 a include aplurality of ribs and grooves 50 b that run completely around theperimeter of sealing flange 50 a to ensure sealing without the use ofsealant. As shown, bladder cover plate 51 is installed with a portion ofthe screws 37 that are used to join housing cover plate 33 to mainhousing 32 and with additional screws 52 that engage sealed bosses 33 cformed in housing cover plate 33.

Internal expansion bladder 50 allows expansion of sump 55 fluid withinthe sealed transmission assembly 30 without the need for a vent toatmosphere or an external expansion tank with associated fittings, etc.,while enabling the versatility of orienting transmission assembly 30 inany operating position desired based upon the vehicle or equipmentconfiguration to which transmission assembly 30 is to be applied.However, as is known in the art, check plugs, such as check plugs 63 oftransmission assembly 30, must remain submerged in sump 55 fluid at alltimes during operation of transmission assembly 30. If transmissionassembly 30 is installed in an orientation with the output shaft 36pointing downward such that check plugs 63 are oriented at the upwardend of the installed transmission assembly 30 (i.e., if transmissionassembly 30 as shown in FIG. 8 is rotated approximately 90 degreescounter-clockwise and installed in a vehicle, for example), thensubstantially complete filling of sump 55 with fluid and purging of airfrom the internal volume of transmission assembly 30 must be ensuredduring the manufacturing process. If it is deemed by a manufacturer oftransmission assembly 30 to be impractical to reliably ensure properfluid fill level at a cost and confidence level acceptable to thatmanufacturer, then modification of main housing 32 to increase theinternal volume of main housing 32 and the sump 55 fluid volume in thevicinity of the check plugs 63 could be deemed desirable or necessary inorder to avoid placing restrictions on the installed orientation oftransmission assembly 30. Also, fluid expansion bladder 50 could berelocated to the end of main housing 32 that is proximate to check plugs63. After sump 55 of transmission assembly 30 is properly filled with anappropriate transmission fluid, a plug 54 is installed in fluid fillopening 32 j of main housing 32 to complete the formation and sealing ofsump 55. Plug 54 may be a threaded plug or a push-in rubber orelastomeric plug, for example. Plug 54 may be positioned in housing 32in a location other than that shown herein, if desired.

As an alternative, housing cover plate 134, as shown in FIG. 5, replaceshousing cover plate 33 and internal expansion bladder 50 of transmissionassembly 30. Housing cover plate 134 includes a plate component 133 andan integral, over-molded bladder-gasket 150 comprising an expansionbladder component 150 a and a gasket component 150 b. Alternatively,expansion bladder component 150 a and gasket component 150 b need not beover-molded onto plate component 133 at the same time or joined togetheras shown, but rather can be over-molded separately onto plate component133 and may each be formed with different material properties to achievedesired flexibility, sealing characteristics, etc.

Bladder 250 as shown in FIGS. 22 and 23 is an alternative design thatcan be used in place of bladder 50 as described above, and flange 250 aoperates similarly to flange 50 a previously described for mountingbladder 250 to the housing. Bladder 250 has an approximately rectangularshape with two longer sidewalls 250 m and two shorter sidewalls 250 nadjacent to surfaces 250 p and 250 f to bound an internal volume, incooperation with a plate such as bladder cover plate 51. Two pairs ofstiffening ribs 250 d and 250 e extend into this internal volume ofbladder 250 perpendicularly from longer side walls 250 m, running alongthe respective longer sidewalls 250 m from flange 250 a to top surface250 p. These ribs 250 d, 250 e assist not only in strengthening the bodyof bladder 250 to prevent unwanted collapse, but also to improve itsability to collapse in a controlled manner as the oil expands. Ribs 250d are preferably longer than ribs 250 e, and it has been determined thatthe placement of the two longer ribs 250 d on one longer sidewall 250 mand the other two shorter ribs 250 e on the opposite longer sidewall 250m improves the ability of bladder 250 to collapse appropriately. Thelocation of the two longer ribs 250 d between the two shorter ribs 250e, as depicted in FIGS. 22 and 23, creates three separate zones 250 g,250 h and 250 i in the internal volume of bladder 250 for sidewalls 250m, 250 n to collapse in on themselves. A beveled surface 250 f isprovided on one sidewall 250 m primarily for clearance so that bladder250 fits within the housing 32, but this beveled surface 250 f alsoimproves the ability of bladder 250 to collapse appropriately. Bladder250 may be composed of various materials but is preferably a rubbercompound. Tab 250 c ensures proper placement of bladder 250 in atransmission assembly.

When housing cover plate 33 or alternate housing cover plate 134 isinstalled onto main housing 32, an integrally-formed bearing supportstructure 33 a or integrally-formed bearing support structure 133 a,respectively, cooperates with a thrust bearing positioning recess 32 kformed in main housing 32 to properly position motor thrust bearing 43.It will be understood that housing cover plate 33 or alternate housingcover plate 143 may use protrusions (not shown) which mate with andengage openings (not shown) in main housing 32 to aid assembly byensuring proper placement of the housing cover plate 33 or 143.

As is evident upon examination of FIGS. 4, 6, 8 and 13, transmissionassembly 30 can be assembled without rotating main housing 32 about morethan one axis during assembly. All mechanical components of transmissionassembly 30 can be assembled into main housing 32 prior to attachinghousing cover plate 33. All mechanical components and, in particular,all rotating shaft bearings and/or bearing surfaces are located withinmain housing 32. This design and arrangement of all rotating componentswithin or at least partially supported by a single housing componentallows all critical housing machining to occur within only one housingcomponent (main housing 32) so that critical alignment of bearingsurfaces located in separate housing components is not required in themanufacture of transmission assembly 30.

Referring primarily to FIGS. 1, 3, 6, 8, 9 and 12, the speed anddirection of the output shaft 36 of transmission assembly 30 iscontrolled by rotating a control arm 31 engaged to trunnion shaft 45 aof trunnion-mounted swash plate 45 to move swash plate 45 in an arcuatemanner along the axis of rotation of trunnion shaft 45 a. Swash plate 45is engaged to pump pistons 40 a in a known manner. The allowed rotationangle of control arm 31 is limited by contact between stops 31 a formedon control arm 31 with protrusion 32 h formed on main housing 32. In theillustrated embodiment, three pairs of stops 31 a are positioned oncontrol arm 31 such that control arm 31 can be attached in one of threeorientations to trunnion shaft 45 a, as permitted by the arrangement ofstops 31 a and the engagement of four tapered ribs 31 b, formed on shaft31 c of control arm 31, with four tapered recesses 45 e formed on adistal end of trunnion shaft 45 a. As shown most clearly in FIG. 6,recesses 45 e and ribs 31 b are generally rectangular in shape.

In order to install swash plate 45, swash plate 45 is tilted and thenmoved into position inside housing 32 so that trunnion shaft 45 a entersbore 32 a as swash plate 45 is placed against cradle bearings 46 and isalso positioned between the two locating thrust ribs 32 c formed inhousing 32. Then, control arm 31 can be oriented to the desired mountingposition and installed, along with seal 47 and seal 48, by means ofscrew 49. Control arm 31 can have one or more leak indicator grooves(not shown) to aid assembly by ensuring proper installation of seal 47and seal 48. As screw 49 is tightened, ribs 31 b engage recesses 45 eand a concentric ridge 45 f formed on the end of trunnion shaft 45 a ispressed against trunnion shaft seal 47 while the control arm shaft seal48 is trapped between shoulder 31 d formed on control arm shaft 31 c andshoulder 32 m formed on main housing 32. Control arm shaft seal 48 maybe omitted and a press-fit lip-seal (not shown) can be used in lieuthereof.

FIGS. 19, 20, and 21 show a portion of an alternative trunnion shaft andcontrol arm assembly. Trunnion shaft 145 a has a plurality of taperedrecesses 145 e formed about its circumference, and which engage taperedribs 131 b of control arm shaft 131 c. In this embodiment, recesses 145e have a pair of side walls 145 h which meet so that tapered recesses145 e are generally triangular in shape; similarly, tapered ribs 131 beach have a similar shape to fit within each tapered recess 145 e. Theshapes and relationships of tapered recesses 145 e and tapered ribs 131b as shown balance calculated force vectors to minimize the creation ofradial force by trunnion shaft 145 a in the vicinity of tapered recesses145 e. Tapered recesses 145 e, along with tapered ribs 131 b, can beclocked (e.g. by 45°) to aid assembly by facilitating installation ofswash plate 145 into main housing 32. It should be understood thatvarious features of the alternative embodiments of components describedherein that do not cooperate with or affect the essential operation ofone another can be used together or separately. For example, swash plate145 and control arm 131 can be used with rotary bypass mechanism 69 and,likewise, swash plate 45 and control arm 31 can be used with bypass arm170 and bypass shaft 171, described below.

Specialized cradle bearings 46 comprising integrally-formed locator pins46 a that engage pin receptacles 32 b formed in main housing 32, aresupportively positioned above trunnion shaft 45 a and trunnionprotrusion 45 b to allow arcuate movement of swash plate 45. Swash plate45 is further supported by the spring forces applied by pump assembly 40against pump thrust bearing 41 that is installed in a positioning recessformed in swash plate 45, as shown in FIG. 8. Swash plate 45 is furthersupported through its previously described attachment to control armshaft 31 c that is integrally formed as part of control arm 31 and isrotatably supported in bore 32 a formed in main housing 32. Excessiveaxial movement of swash plate 45 is prevented by its location betweentwo thrust ribs 32 c that each interface with a cam-like surface 45 gthat is formed with a curvature to interface with the drafted surface ofeach rib 32 c as swash plate 45 is arcuately moved through its fullrange of motion. These interfacing surfaces are formed such that theinterface of a cam-like surface 45 g with a corresponding draftedsurface of a rib 32 c does not cause any significant axial movement ofswash plate 45. The two thrust ribs 32 c are spaced apart such that onlyone rib 32 c can be in contact with one cam-like surface 45 g at anygiven time, thereby allowing free arcuate movement of swash plate 45while limiting the axial movement of swash plate 45. While two ribs 32 care depicted, it will be appreciated by those skilled in the art thatfour ribs could be used such that only two of such ribs can be incontact with one cam-like surface 45 g at any given time.

Positioning of specialized cradle bearings 46 on the trunnion shaft 45 aand trunnion protrusion 45 b allows structural arch ribs 45 c andstructural skirting ribs 45 d to be integrally formed on the symmetricalswash plate 45 so that it is strengthened to a level that enablesfabrication of swash plate 45 as an as-cast component with no machiningrequirement prior to installation in transmission assembly 30. As seenin FIG. 3, skirting ribs 45 d extend in a direction generallyperpendicular (e.g., within about 15°) to the direction of arch ribs 45c.

Referring primarily to FIGS. 1, 3, 6, 8 and 10, a rotary bypassmechanism 69 comprising a bypass arm 70 and a bypass shaft 71 isprovided to allow a vehicle or equipment operator to selectivelydisconnect the hydraulic fluid circuit of transmission assembly 30 byunseating the check balls 63 a of check plugs 63 that are installed inports 38 k formed in the ends of fluid passage tubes 38 j of centersection 38. If hydraulic pressure alone is used to bias the check balls63 a to the seated position shown in FIG. 3, the longitudinal axis ofcheck plugs 63 should be horizontal so that gravity will not unseatcheck balls 63 a. Check plugs of the type shown and described herein arealso known as check valves and well known in the art and will not bedescribed in greater detail herein.

Bypass arm 70, which can be composed of a plastic material, comprisestwo stops 70 a, each serving as a boundary at opposite ends of aflexible arch 70 b that is formed adjacent to a material void or opening70 c formed in bypass arm 70, with the material void or opening 70 callowing arch 70 b to flex as will be further described. Bypass arm 70also includes two recesses 70 d, with one recess 70 d located at eachend of the flexible arch 70 b and therefore adjacent to each of the twostops 70 a. This arrangement of features provides a detent function ateach end of flexible arch 70 b. When bypass arm 70 is positioned in afirst detent position, as shown most clearly in FIGS. 1 and 3, fingers71 a formed on shaft 71 are not in contact with the check balls 63 a ofcheck plugs 63. As the bypass arm is pivoted, flexible arch 70 b wipesagainst projection 32 i formed on housing 32, providing rotationalresistance of shaft 71. When this rotational resistance is overcome,bypass arm 70 is rotated to a second detent position in which the twofingers 71 a formed on shaft 71 simultaneously unseat the two checkballs 63 a of the two check plugs 63. In each of the two detentpositions, projection 32 i is seated in a recess 70 d. Thisconfiguration enables the use of a very simple push-pull linkage orcable mechanism, or even rotation of bypass arm 70 by hand (if bypassarm 70 is easily accessible and its rotational resistance is not toogreat) to engage and disengage the bypass function.

As can be seen in FIGS. 1 and 6, the bypass arm 70 can be easilyinstalled onto bypass shaft 71 by pressing it into position so that itis retained on shaft 71 by a snap fit end 71 e formed on shaft 71.Similarly, bypass arm 70 can be removed from bypass shaft 71 by pinchingthe snap fit end 71 e while pulling the bypass arm away from mainhousing 32 along the rotational axis of bypass shaft 71.

As shown in FIGS. 1, 3 and 13, the bypass shaft 71 also comprisesintegrally formed bearing surfaces 71 b and 71 c that are supported,respectively, in an opening and a recess formed in shaft supports 32 vand 32 w that are formed on opposing sides of main housing 32. A sealabutment flange 71 d formed on bypass shaft 71 ensures limited outwardmovement along the rotational axis of bypass shaft 71 because of itslocation adjacent to the bypass shaft seal 72.

FIGS. 16, 17, and 18 show a portion of an alternative embodiment of thebypass arm mechanism. Bypass arm 170, which can be composed of a plasticmaterial, has two pairs of detent openings, 170 d and 170 e, which matewith and alternately engage detents 132 z. When bypass arm 170 ispositioned in a first detent position as shown in FIG. 16, detents 132 zengage detent openings 170 d. When bypass arm 170 is manipulated bypivoting bypass arm 170 to a second detent position as shown in FIG. 17,detents 132 z engage detent openings 170 e. Protrusion 132 i stops thepivoting of bypass arm 170 when the second detent position is reached.

Referring to FIG. 18, manipulating bypass arm 170 causes bypass shaft171 to rotate because bypass arm 170 is fixed to bypass shaft 171 byfastener 174. Shoulder 171 g contacts bypass arm 170 to position same.Wave spring 175 bears against flange 171 f and bypass shaft seal 172,which are all retained by retaining ring 173. The force of wave spring175 against flange 171 f and seating of detents 132 z in the pairs ofdetent openings 170 d and 170 e provides resistance of bypass shaft 171to rotation. Bypass shaft 171 is partially supported by contact ofbearing surface 171 b with main housing 132. Bypass shaft 171 is alsopartially supported by contact with main housing 132 of a bearingsurface (not shown) of bypass shaft 171 corresponding to bearing surface71 c of bypass shaft 71.

As can be seen most clearly in FIGS. 7 and 8, the rotational axis 35Axof input shaft 35, the rotational axis 36Ax of output shaft 36, and therotational axis 45Ax of trunnion shaft 45 a that is integrally formed aspart of swash plate 45 all lie on a single vertical plane with one endof both the input shaft 35 and output shaft 36 supported in centersection 38 and the opposite end supported in main housing 32. Thecomponents of the hydraulic circuit are all arranged symmetrically aboutthe plane on which these three axes are located, resulting in asubstantially symmetrical main housing 32 and transmission assembly 30.With a few obvious and relatively minor modifications, transmissionassembly 30 can be configured symmetrically in its entirety about thevertical plane represented by line 8-8 of FIG. 7. It will be understoodthat this plane need not be vertical with respect to ground astransmission assembly 30 may be mounted in different configurations suchas is shown in vehicle 10 of FIG. 14, where the plane would behorizontal with respect to ground, as compared to vehicle 80 of FIG. 15where the plane would be vertical.

Referring primarily to FIGS. 7, 8, 10, 11 and 12, vertical input shaft35 passes through opening 32 n of main housing 32 and is supported by abearing 58 and by a journal opening 38 c formed in center section 38.Vertical input shaft 35 is drivingly engaged to the axial piston pumpassembly 40 that is rotatably disposed on pump running surface 38 a ofcenter section 38. The horizontal output shaft 36 passes through opening32 p of housing 32 and is supported by a bearing 59 and by a journalopening 38 d formed in center section 38. Horizontal output shaft 36 isdrivingly engaged to the axial piston motor assembly 42 that isrotatably disposed on motor running surface 38 b of center section 38.Vertical input shaft 35 is restrained from excessive axial movement inone direction by shoulder 35 a formed on shaft 35 and in the oppositedirection by center section 38 thrust surface 38 m. In this manner,input shaft 35 is trapped in position during assembly without usingadditional parts, such as retaining rings, for example. Similarly,output shaft 36 is restrained from excessive axial movement in onedirection by shoulder 36 a formed on shaft 35 and in the oppositedirection by a simple wire retaining ring 65 or other similar retainingring.

Referring primarily to FIGS. 8 and 10-13, center section 38 includeshydraulic porting and passages for hydraulically connecting pumpassembly 40 and motor assembly 42. Pump running surface 38 a comprisestwo kidney-shaped hydraulic fluid ports 38 e and motor running surface38 b comprises two kidney-shaped hydraulic fluid ports 38 f. Two checkplug ports 38 k are provided in the ends of the two substantiallyparallel fluid passage tubes 38 j to receive the two check plugassemblies 63. The fluid passage tubes 38 j provide fluid communicationbetween pump running surface 38 a and motor running surface 38 b.

Precision installation of center section 38 is accomplished by employinga combination of design elements and installation techniques. Assemblytools (not shown) comprising pin locators (not shown) accuratelyposition the center section 38 in main housing 32 by simultaneouslyengaging two machined openings 38 g formed in center section 38 and twoaxially aligned machined recesses 32 q formed in main housing 32 whileinstalling the four center section attachment screws 39 through fourmachined openings 38 p formed in center section 38 and into engagementwith four machined recesses 32 t formed in main housing 32. Machinedsurfaces 38 n formed on two alignment protrusions 38 i integrally formedon center section 38 interface with two machined vertical surfaces 32 sformed in main housing 32 to further ensure proper alignment of centersection 38. Finally, four machined mounting boss faces 38 h formed oncenter section 38 interface with four machined mounting boss faces 32 rformed on main housing 32, with these machined mounting boss faces alllocated on a single horizontal plane at installation to help ensurecomponent machining and mating accuracy. This combination of designelements and installation techniques for aligning and installing centersection 38 allows screws 39 to be self-tapping screws (i.e.thread-cutting or thread-forming screws), thereby eliminating a tappingprocedure from the machining requirements that are employed tomanufacture the four machined recesses 32 t formed in main housing 32.This combination of design elements and installation techniques doesnot, however, preclude pre-tapping of machined recesses 32 t. Centersection 38 and housing 32 are generally symmetrical about a planepassing through the longitudinal axis of rotation of output shaft 36, asseen most clearly in FIG. 13, resulting in a more compact unit.

Referring primarily to FIGS. 1 and 2, a pulley and fan assembly 44 maybe secured to the input shaft 35 of transmission assembly 30, as shown,to accept a drive belt and provide air flow to cool transmissionassembly 30 when it is belt-driven. Alternatively, transmission assembly30 may be driven directly by coupling a prime mover output shaft (notshown) or other drive shaft (not shown) to input shaft 35, or cooled byother means, such as a separate fan (not shown), when cooling isrequired. Modification of input shaft 35 may be required to accommodatedirect coupling. Various shaft coupling methods are well known and willnot be described in detail herein.

Referring primarily to FIGS. 1, 2 and 7, two mounting bosses 32 d withhole centerlines located on a single vertical plane at a first end oftransmission assembly 30, and at least one mounting lug 32 e with a holecenterline located on a horizontal plane at an opposing end oftransmission assembly 30, are provided to secure transmission assembly30 to a vehicle or equipment frame structure. As shown in FIG. 7,alternatively positioned or additional lugs, such as lug 32 f and lug 32g, both of which are illustrated using phantom lines, may be providedfor additional mounting support or ease of installation of transmissionassembly 30. Alternatively, mounting lugs 32 d, 32 e, 32 f, and 32 g maybe omitted and a plurality of screws 37 replaced with studs (not shown)to attach transmission assembly 30 to a mounting plate of a vehicle (notshown).

As illustrated in FIG. 14, an exemplary wheeled leaf blowing machine orvehicle 10 comprises a prime mover 11 that drives a blower assembly 12and a transmission drive belt 13 that engages pulley and fan assembly 44to drive transmission assembly 30. The speed and rotation direction ofoutput shaft 36 of transmission assembly 30, and therefore, the speedand direction of vehicle 10, is controlled by the operator of vehicle10. Linkage assembly 14 connects an operator control lever 15 to thecontrol arm 31 of transmission assembly 30. Output shaft 36 is drivinglyengaged to a reduction gear set or gear train 16 which may include aroller chain, sprockets and gears, as shown, to drive an axle 17 thatdrives a pair of wheels 18. A bypass linkage 19 is provided to allow theoperator to effectively disconnect the hydraulic fluid circuit in orderto move vehicle 10 without the resistance associated with hydraulicallydriving the transmission. In this vehicle embodiment, prime mover 11,blower assembly 12, transmission assembly 30 and axle 17 are allsecurely attached to or mounted on a vehicle frame 20. As is well known,configurations of operator controls, control linkages, reduction gearsets or gear trains, vehicle frames, etc., can vary widely among vehicleand equipment configurations and manufacturers.

As illustrated in FIG. 15, an exemplary mowing machine or vehicle 80comprises a prime mover 81 that indirectly drives one or more blades 82a of mowing deck assembly 82 and also indirectly drives transmissionassembly 30 by means of a pulley and belt system 83 comprising deckdrive belt 83 a and transmission drive belt 83 b, respectively. A clutchor other known mechanism (not shown) may be used to selectively engageand disengage the blade(s) 82 a of mowing deck assembly 82. As in theexemplary leaf blowing machine described previously herein, the speedand rotation direction of output shaft 36 of transmission assembly 30,and therefore, the speed and direction of vehicle 80, is controlled bythe operator of vehicle 80. Linkage assembly 84 connects an operatorcontrol lever 85 to the control arm 31 of transmission assembly 30. Aspring-operated return-to-neutral (RTN) mechanism 84 a may be providedas part of linkage assembly 84 to return the control arm 31 oftransmission assembly 30 to a neutral position when the operatorreleases lever 85. Output shaft 36 of transmission assembly 30 isdrivingly engaged to a reduction gear set or gear train 86 which mayinclude a roller chain, sprockets and gears, as shown, to drive axle 87that drives a pair of wheels 88. A bypass linkage 89 is provided toallow the operator to conveniently bypass (or effectively disconnect)the hydraulic fluid circuit in order to move vehicle 80 without theresistance associated with hydraulically driving the transmission. Inthis vehicle embodiment, prime mover 81, transmission assembly 30 andaxle 87 are all securely attached to or mounted on a vehicle frame 90.As mentioned previously herein, configurations of operator controls,control linkages, reduction gear sets or gear trains, vehicle frames,etc., can vary widely among vehicle and equipment configurations andmanufacturers.

It will be understood that RTN mechanism 84 a can be omitted and analternate RTN mechanism (not shown) that includes a control arm (notshown) mounted on a transmission assembly that can be very similar inform to transmission assembly 30 (with relatively minor modifications tohousing 32) may be used to return the transmission to a neutral positionwhen the operator releases lever 85. An example of an RTN mechanism(including control arm) of this type is described in commonly-owned U.S.Pat. No. 6,968,687, the terms of which are incorporated herein byreference.

It will also be understood that the pulley and belt system can beomitted either partially or entirely and a transmission assembly 30 ortransmission assembly very similar to transmission assembly 30 (withinput shaft modification as needed) can be driven directly by the primemover or a shaft driven by the prime mover. A mowing deck, for example,or other implement or apparatus, can also be driven by a shaft driven bythe prime mover. Prime mover 11 or prime mover 81 could be an internalcombustion engine, electric motor or the like.

Additional controls (not shown) may be provided to control the start-upand speed of prime mover 11 or prime mover 81, or a prime mover in analternative embodiment (as described herein, but not shown), control apower take-off device (not shown) or control other functions ofvehicles, machines or apparatuses to which transmission assembly 30 isapplied.

While specific embodiments of the invention have been described indetail, it will be appreciated by those skilled in the art that variousmodifications and alternatives to those details could be developed inlight of the overall teachings of the disclosure. Accordingly, theparticular arrangements disclosed are meant to be illustrative only andnot limiting as to the scope of the invention which is to be given thefull breadth of the appended claims and any equivalent thereof.

We claim:
 1. A hydraulic drive device, comprising: a housing forming aninternal sump; a pump running surface disposed in the housing; a pumpcylinder block rotatably disposed on the pump running surface and havinga plurality of pump pistons disposed therein, the pump cylinder blockhaving a first axis of rotation; a thrust bearing engaged to theplurality of pump pistons; a swash plate disposed in the housing andhaving a second axis of rotation perpendicular to the first axis ofrotation, the swash plate comprising a plurality of contact surfaces;and the housing having a plurality of thrust ribs formed on an innersurface thereof, a first of the thrust ribs being located adjacent toone of the plurality of contact surfaces on the swash plate and a secondof the thrust ribs being located adjacent to another of the plurality ofcontact surfaces on the swash plate, the thrust ribs being located sothat at least one, but fewer than all, of the thrust ribs can be incontact with its respective contact surface on the swash plate at atime, wherein the plurality of thrust ribs permits rotation of the swashplate while limiting movement of the swash plate in an axial directionparallel to the second axis of rotation.
 2. The hydraulic drive deviceof claim 1, wherein the first of the thrust ribs has a first draftedsurface and the second of the thrust ribs has a second drafted surface,and each contact surface on the swash plate is formed with a curvaturesuch that interaction of each contact surface with its respectivedrafted surface does not cause significant movement of the swash platein the axial direction of the second axis of rotation.
 3. The hydraulicdrive device of claim 1, wherein the swash plate further comprises amain body and a first structural rib and a second structural rib formedon the main body, each structural rib extending in a direction generallyparallel to the second axis of rotation, wherein one of the plurality ofcontact surfaces is formed on one end of the first structural rib and asecond of the plurality of contact surfaces is formed on one end of thesecond structural rib.
 4. The hydraulic drive device of claim 3, whereinthe swash plate further comprises a pair of skirting ribs formed on themain body, wherein the pair of skirting ribs are located on oppositesides of the swash plate and extend outwardly from the main body in adirection generally perpendicular to the first structural rib and thesecond structural rib.
 5. The hydraulic drive device of claim 4, furthercomprising a pair of cradle bearings disposed in the housing, whereinthe swash plate further comprises a trunnion shaft at a first end of themain body and a trunnion protrusion at a second end of the main bodyopposite the first end, and a first of the cradle bearings engages thetrunnion shaft and a second of the cradle bearings engages the trunnionprotrusion.
 6. The hydraulic drive device of claim 1, furthercomprising: a center section disposed in the internal sump, wherein thepump running surface is formed on the center section and the centersection further comprises a motor running surface; and a hydraulic motordisposed on the motor running surface and comprising a motor cylinderblock driving a motor shaft having a third axis of rotation, wherein thefirst axis of rotation, the second axis of rotation, and the third axisof rotation all lie in a single plane; and a trunnion shaft engaged toand extending from the swash plate, wherein the swash plate issymmetrical about the single plane and the trunnion shaft is disposeddirectly above the motor shaft.
 7. The hydraulic drive device of claim1, further comprising: a trunnion shaft having a first end engaged tothe swash plate, and a second end disposed so as to be accessible fromoutside the housing through a bore formed in the housing; and a controlarm for rotating the trunnion shaft and the swash plate about the secondaxis of rotation, the control arm having a shaft portion extending intoand engaging the bore, wherein the shaft portion supports the second endof the trunnion shaft within the bore.
 8. The hydraulic drive device ofclaim 7, wherein the swash plate further comprises a main body and afirst structural rib and a second structural rib formed on the mainbody, each structural rib extending in a direction generally parallel tothe second axis of rotation, wherein one of the plurality of contactsurfaces is formed on one end of the first structural rib and a secondof the plurality of contact surfaces is formed on one end of the secondstructural rib.
 9. The hydraulic drive device of claim 8, wherein theswash plate further comprises a pair of skirting ribs formed on the mainbody, wherein the pair of skirting ribs are located on opposite sides ofthe swash plate and extend outwardly from the main body in a directiongenerally perpendicular to the first structural rib and the secondstructural rib.
 10. The hydraulic drive device of claim 1, furthercomprising a pair of cradle bearings disposed in the housing, whereinthe swash plate further comprises a main body, a trunnion shaft at oneend of the main body and a trunnion protrusion at another end of themain body opposite the trunnion shaft, and a first of the cradlebearings engages the trunnion shaft and a second of the cradle bearingsengages the trunnion protrusion.
 11. The hydraulic drive device of claim10, wherein each cradle bearing further comprises at least one locatorpin that engages a corresponding pin receptacle formed in the housing toposition the cradle bearing.
 12. A swash plate for use with a hydraulicdrive device having a rotating axial piston unit, the swash platecomprising: a main body having a first end and a second end, wherein themain body is rotatable about an axis of rotation that extends betweenthe first end and the second end; a thrust bearing engaged to the mainbody of the swash plate for contacting a set of pistons of the rotatingaxial piston unit; spaced-apart first and second structural ribs locatedon and extending along a length of the main body, the first structuralrib located on a first side of the main body and the second structuralrib located on a second side of the main body; and spaced-apart firstand second skirting ribs located on and extending along the length ofthe main body, the first skirting rib located on the first side of themain body and the second skirting rib located on the second side of themain body.
 13. The swash plate of claim 12, wherein the swash plate issymmetrical about a plane that extends from the first end to the secondend of the main body and encompasses the axis of rotation.
 14. The swashplate of claim 12, wherein the first skirting rib extends outwardly fromthe first side of the main body in a direction generally perpendicularto the first structural rib and the second skirting rib extendsoutwardly from the second side of the main body in a direction generallyperpendicular to the second structural rib.
 15. The swash plate of claim14, further comprising a trunnion shaft having a distal free end and aproximal end connected to the first side of the main body so thatrotation of the trunnion shaft rotates the main body to control outputof the rotating axial piston unit, and a trunnion protrusion connectedto the second side of the main body.
 16. The swash plate of claim 14,wherein the first and second structural ribs and the first and secondskirting ribs each extend outwardly from the main body in a directiongenerally parallel to the axis of rotation.
 17. The swash plate of claim14, wherein the swash plate is symmetrical about a plane that extendsfrom the first end to the second end of the main body and encompassesthe axis of rotation.
 18. A hydraulic drive device, comprising: ahousing forming an internal sump; a hydraulic pump disposed in thehousing and comprising a plurality of axial pistons; a swash platedisposed in the housing and engaged to the plurality of axial pistonsfor varying an output of the hydraulic pump, the swash plate having afirst end and a second end; a trunnion shaft having a proximal endengaged to the first end of the swash plate, and a distal free enddisposed so as to be accessible from outside the housing through a boreformed in the housing; and a control arm for rotating the trunnion shaftand the swash plate about a first axis of rotation, the control armhaving a shaft portion extending into and engaging the bore, wherein theshaft portion supports the distal free end of the trunnion shaft withinthe bore.
 19. The hydraulic drive device of claim 18, further comprisinga trunnion protrusion engaged to the second end of the swash plate andsupported in the housing.
 20. The hydraulic drive device of claim 19,further comprising: a hydraulic motor disposed in the housing andhydraulically connected to the hydraulic pump; a motor shaft engaged toand driven by the hydraulic motor and having a second axis of rotationthat is parallel to the first axis of rotation, wherein the motor shaftis disposed directly below the trunnion shaft.
 21. The hydraulic drivedevice of claim 19, wherein the swash plate comprises a plurality ofcontact surfaces formed thereon, and the housing comprises a pluralityof thrust ribs formed on an inner surface thereof, a first of the thrustribs being located adjacent to one of the plurality of contact surfaceson the swash plate and a second of the thrust ribs being locatedadjacent to another of the plurality of contact surfaces on the swashplate, the thrust ribs being located so that at least one, but fewerthan all, of the thrust ribs can be in contact with its respectivecontact surface on the swash plate at a time, wherein the plurality ofthrust ribs permits arcuate rotation of the swash plate while limitingmovement of the swash plate in an axial direction parallel to the firstaxis of rotation.
 22. The hydraulic drive device of claim 18, furthercomprising a seal, wherein the seal and the shaft portion of the controlarm cooperate to close and seal the bore.